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Whereas biphenyl‐4,4′‐dicarbonyl dichloride reacts with methylenetriphenylphosphorane to give the expected bis(acylylide) 1, the same reaction of biphenyl‐2,2′‐dicarbonyl dichloride results in a multistep rearrangement leading to the zwitterionic 2‐[1,3‐bis(triphenylphosphonio)prop‐1‐en‐3‐id‐2‐yl]biphenyl‐2′‐carboxylate 2. X‐ray crystal structures of both 1 and 2 are reported.

Oxo-stabilised phosphonium ylides are found to form crystalline hydrogen-bonded adducts with aromatic carboxylic acids, as confirmed by X-ray diffraction. There is also strong hydrogen bonding in solution as indicated by 13C NMR spectroscopy and this confirmed adduct formation with acetic acid, benzamide, thiobenzamide, benzyl alcohol, benzenesulfinic acid and diphenylphosphinic acid. The X-ray structure of the benzamide adduct was also determined, showing a hydrogen-bonded dimeric structure. A bis(stabilised ylide) was also prepared and is found to form a complex hydrogen-bonded adduct with benzoic acid, ethanol and water.

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